This invention relates generally to hermetic feedthrough terminal subassemblies and related methods of construction, particularly of the type used in active implantable medical devices such as cardiac pacemakers, implantable defibrillators, cochlear implants, neurostimulators, active drug pumps, and the like, and those incorporating an electromagnetic interference (EMI) filter designed to decouple and shield undesirable EMI signals from an associated device. More particularly, the present invention relates to an improved hermetic terminal that includes bonding pads for convenient attachment of lead wires by way of thermal or ultrasonic bonding, soldering or the like. The bonding pads can be attached to the capacitor structure or to a terminal pin.
Feedthrough terminal assemblies are generally well known for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. In a cardiac pacemaker, for example, the feedthrough terminal pins are typically connected to one or more lead wires within the case to conduct pacing pulses to cardiac tissue and/or detect or sense cardiac rhythms.
However, the lead wires can also undesirably act as an antenna and thus tend to collect stray electromagnetic interference (EMI) signals for transmission into the interior of the medical device. Studies conducted by the United States Food and Drug Administration, Mt. Sinai Medical Center and other researchers have demonstrated that stray EMI, such as RF signals produced by cellular telephones, can seriously disrupt the proper operation of the pacemaker. It has been well documented that pacemaker inhibition, asynchronous pacing and missed beats can occur. All of these situations can be dangerous or life threatening for a pacemaker-dependant patient. In prior devices, such as those shown in U.S. Pat. Nos. 5,333,095 and 4,424,551 (the contents of which are incorporated herein), the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple electromagnetic interference (EMI) signals to the equipotential housing of the medical device.
In general, the ceramic feedthrough capacitor which has one or more passages or feedthrough holes is connected to the hermetic terminal of the implantable medical device in a variety of ways. In order for the EMI filter feedthrough capacitor to properly operate, a low impedance and low resistance electrical connection must be made between the capacitor ground electrode plate stack and the ferrule, which, in turn, mechanically and electrically connects to the overall conductive housing of the implantable medical device. For example, in a cardiac pacemaker, the hermetic terminal assembly consists of a conductive ferrule generally made of titanium which is laser-welded to the overall titanium housing of the implantable medical device. This not only provides a hermetic seal but also makes the ferrule of the hermetic terminal a continuous part of the overall electromagnetic shield that protects the electronics of the implantable medical device from electromagnetic interference. The ceramic feedthrough capacitor is, in turn, electrically and mechanically bonded to the ferrule of said hermetic terminal. In the past, and, in particular, as described in U.S. Pat. Nos. 5,333,095 and 4,424,551, the connection is typically performed using a thermal setting conductive adhesive. One such material is a silver flake loaded conductive polyimide, silver flakes overlay each other, increasing their flake-to-flake contact area, and having a much lower inductance and lower resistance at high frequency.
FIG. 1 is a cut away perspective view of a prior art unipolar ceramic feedthrough capacitor 110. This capacitor 110 has a conventional external ground 112 formed by the conductive termination around its outside diameter. This is a conductive termination which would be electrically connected to the ferrule of the hermetic terminal of an implantable medical device. The inside diameter hole 114 is also metallized 116 for electrical connection to the lead wire that passes through the center passageway 114. One can see in the cut away the active 118 and ground 120 electrode plate sets. Feedthrough capacitor geometry is highly preferable for EMI filters in that it acts as a coaxial broadband transmission line filter. This means that a feedthrough capacitor offers effective attenuation over a very broad range of frequencies without the series resonance problem that plagues conventional rectangular monolithic ceramic chip capacitors.
FIG. 2 is the schematic diagram of the feedthrough capacitor of FIG. 1.
FIG. 3 is a cross-section drawing which illustrates the feedthrough capacitor 110 of FIG. 1 installed to the hermetically sealed ferrule 122 of a housing 124 of an implantable medical device in accordance with U.S. Pat. No. 5,333,095, entitled FEEDTHROUGH FILTERED CAPACITOR ASSEMBLY FOR HUMAN IMPLANT. This device is also referred to as a unipolar (one lead wire) EMI filtered hermetic terminal. It is also known as a one section single element EMI filter. The schematic diagram for the filter is shown in FIG. 4. It is possible to have multielement EMI filters (combinations of inductors and capacitors) with a single (unipolar) lead wire, or have multiple lead wires with a single element EMI filter (feedthrough capacitor only). The connection between the outside diameter metallization 112 of the feedthrough capacitor 110 and the ferrule 122 is accomplished with a thermal setting conductive adhesive 126. In the preferred embodiment, connection 126 is typically not a continuous connection 360 degrees around the entire outside diameter of the ceramic capacitor 110. The electrical connection material 126 is usually discontinuous to allow for helium leak detection and also to minimize thermal and mechanical stresses to the capacitor 110.
The capacitor 110 is surface mounted and bonded to the ferrule 122 of the hermetic terminal using an adhesive backed polyimide supported washer 128, which is further described in FIG. 6. The hermetic terminal of FIG. 3 is formed by gold brazes 130 and 132. Braze 130 makes a 360 degree mechanical and hermetic seal between the ferrule 122 and the alumina ceramic insulator 134. Gold braze 132 forms a 360 degree mechanical and hermetic seal between the lead wire or terminal terminal pin 136 and the alumina ceramic terminal 134. The capacitor ground electrode plates 120 are connected in parallel to the capacitor outside termination 112. The capacitor ground electrode plates 120, in turn, are connected to the ferrule 122 by way of the electrical connection material 126 disposed between the capacitor metallization 112 and the surface of the ferrule 122. In a typical medical implant EMI filter, the material 126 is of the group of solder, braze, or a thermal setting conductive polymer such as conductive polyimide or conductive epoxy. The electrical connection is made between the capacitor inside diameter metallization 116 and the terminal terminal pin 136 with connection material 138, which is typically of the same material described above with respect to connection material 126. If the terminal terminal pin 136 is of solderable material, which, for human implant applications, includes the group of platinum and platinum iridium biocompatible alloys, then material 138 can be solder, conductive thermal setting adhesives or the like. However, in the case where the terminal terminal pin 136 is of niobium, tantalum or titanium, solders and conductive adhesives generally cannot be applied directly to such pin materials. In this case, the terminal pin 136 would need pretreatment in order to eliminate contact problems associated with high resistance surface oxides.
The ceramic capacitor 110 is often comprised of relatively weak barium titanate, strontium titanate or equivalent high K dielectric. As a general rule, as one raises the dielectric constant, K, of a ceramic material, the structurally weaker it becomes. Leads extending from the circuitry of the implantable device to the feedthrough assembly, or those leads extending from the implantable device to the organ or another device, must be connected to the feedthrough terminal assembly. However, during ultrasonic or thermal wire bonding, considerable energy is imparted into the structure, which can damage the structure, and particularly the ceramic capacitor.
Moreover, for implantable medical devices, it is generally required that any of the electrical circuit connections that are in series with the input or output of the device should be of highly reliable connections. For example, in a cardiac pacemaker, the lead wires that are implanted in the heart sense both biologic electrical signals and also provide pacing pulses to correct cardiac arrhythmias. It is generally not acceptable to have an opening or break in this lead wire anywhere in the system that would then be reattached during initial manufacturing with solder, conductive thermal setting adhesives or the like.